The POSCAR file:                          

 

The POSCAR file defines your crystal structure.  Create a POSCAR file based on the known crystal structure and lattice parameters.  You can get crystal structure information from experiments or other calculations.  I like to use http://cst-www.nrl.navy.mil/lattice/ .

 

MgO has the ROCKSALT (NaCl) structure http://cst-www.nrl.navy.mil/lattice/struk/b1.html

We can use the information given on the website to set up the crystal structure.  For now, we guess at the lattice parameter based on the information available.  VASP will relax the structure and find the lowest energy lattice parameter in the calculation.

 

$ cat POSCAR

MgO                                    

5.63

0.0 0.5 0.5

0.5 0.0 0.5

0.5 0.5 0.0

1 1

Direct

0.0 0.0 0.0

0.5 0.5 0.5

 

Line 1 - Title. Not read in by VASP

Line 2 - lattice parameter (in Angstroms)

Lines 3,4,&5 - lattice vectors that describe the FCC structure.  Make sure you can draw out

      these lattice vectors and understand how it forms the Rocksalt lattice. 

Line 6 - Describes how many atoms of each type there are.  Here there is 1 atom of type 1

      and 1 atom of type 2. The type is defined the POTCAR file. 

      Here, type 1 =Mg and type 2 =Oxygen. 

Line 7 - Specify Direct or Cartesian coordinates

Line 8 - Atomic position for the first atom (on line 6). This atom is at 0 0 0. 

      The position on line 8 is translated by the vectors given on lines 3,4&5.

Line 9 - Atomic position for the second atom atom (on line 6). This atom is at 0.5 0.5 0.5. 

 

Make sure you understand how the POSCAR file creates the Rocksalt structure!

 

To visualize this file you can create an xyz file and look at it in a visualization program. 

First, name each atom in the POSCAR file:

convasp -numnames Mg O < POSCAR > POSCAR.n

 

Now create an xyz file:

convasp -xyz 4 4 4  < POSCAR.n > POSCAR.xyz

 

Then import POSCAR.xyz into a visualizer.

To set up a different structure, you create a different POSCAR file for that structure.  For example, if you had two atoms - both of different types, the basic idea of the POSCAR file would look like:

 

name of file                                      

lattice parameter    

lattice vector 1

lattice vector 2

lattice vector 3

1 1

Direct

starting position for atom 1 (of type 1)

starting position for atom 2 (of type 2)

 

Note: VASP has periodic boundary conditions, so even though we are only specifying 2 atoms above, VASP views it as a bulk sample.  In order to make a surface, you have to add space for vacuum into your POSCAR file.

 

* Extra Practice:

 

To gain familiarity with POSCAR files, create the following POSCAR files:

1. FCC Iron

2. BCC Iron

3. BCC Co Fe with 25% Fe

4. BCC CoFe with 50% Fe using ABAB stacking along the 1 0 0 direction.

5. BCC CoFe with 50% Fe using AABB stacking along the 1 0 0 direction.

6. An FCC surface (for any atom you choose).

Any other group member that uses DFT codes will be able to tell you if your POSCAR files are correct.